The goal of this proposal is to determine incidence and risk factors for HIV infection in rural and urban populations in Malawi; both are relevant populations for HIV vaccine trials. The diverse elements necessary to achieve this aim are described, and the 5 components of the RFA are addressed. The experience and established infrastructure of a large mother-to-infant HIV transmission study, which we have been conducting in Malawi since 1989, will be used to investigate the following factors in male rural workers and in urban women: 1) the rate of new HIV-1 infection in male sugar estate workers and urban antenatal women, 2) the impact of STD control on new HIV-1 infection in male sugar estate workers, and 3) the co-factors of HIV-1 seroconversion in urban women and of HIV-1 transmission from infected mothers to their babies. Our studies of urban women in Blantyre have established the HIV-1 seroprevalence curve between 1985 and 1991, the seroconversion rate between 1989 and 1991, and the contribution of STDs and other risk factors to HIV-1 infection. Most of the population in Malawi, however, lives in semi-urban and rural areas, and this proposal addresses the need to investigate rural inhabitants for the specific purpose of HIV vaccine trial preparation. In this study, we will determine the rate of HIV acquisition in sugar estate workers at two different estates of the same corporation, as well as establish whether STD control combined with AIDS education/condom promotion can significantly reduce the rate of HIV-1 seroconversion, compared to AIDS education/condom promotion alone. STD intervention studies have a potentially large impact on HIV transmission, and studies of risk factors that include intervention components (e.g. STD treatment) are more acceptable in many countries dealing with the HIV epidemic. The organized institutional structure of the sugar estates will be a critical asset to maximize the follow-up rate of study participants. Also enrolled will be a cohort of HIV-1 seronegative urban women, a population will considerably different epidemiologic characteristics from male sugar estate workers. The findings on co-factors of HIV-1 seroconversion and of viral transmission from mother to infant will provide detailed information on a large group of women. Follow-up will be facilitated and improved by using the infrastructure of the ongoing study and by strengthening the established collaboration with the Ministry of Health and with the newly created Malawi College of Medicine.